1. Field of the Invention
The invention relates to the technical field of wireless networks and, more particularly, to a system and method for reducing call establishment delay in a wireless network.
2. Description of Related Art
With extensive implementation of wireless networks (such as 802.11/WLAN, 802.16/WiMAX) in a public network or enterprise network, a wireless network environment can provide a packet voice service, voice over IP (VoIP), or multimedia service to a personal communication as a cheap and convenient choice. Most Internet communication systems use a session initiation protocol (SIP) or similar signaling protocol as a communication protocol standard to establish a connection. As shown in FIG. 1, the configuration of a wireless communication system uses an IP network or the Internet as the core network 10, and also uses an extension of wireless network access point/base stations (base stations) 11 to provide the wireless network access capability. One or more servers 12 are implemented in the core network 10 to assist in establishing a communication connection or session. The communication connection can be a voice over IP, video over IP, or other multimedia connection. With the help of the server 12, the Internet or IP node 13 can establish a connection communication service through the signaling protocol and the wireless terminal 15 in the wireless network.
However, the signaling protocol typically is based on a fixed network in design and does not consider the features of wireless network environments. As such, for a connection establishment, the signaling protocol typically supposes that the network is a wired network. When receiving a connection request from a caller in the wired network, the server 12 immediately forwards the call to the callee. In case of no immediate response, the server 12 further supposes that the wired network has a collision or the relative node is closed. In this case, the server 12 retransmit the call to the callee after starting a waiting mechanism for a span. Such a design of reducing network throughput and collision by prolonging the retransmission time is presented frequently in the signaling of the wired network. However, in a wireless network environment, a wireless node may not correctly receive partial downlink data due to power-saving design or temporary setup disconnection to the base station. Under the cited time-prolonging design, such a feature may cause the initial time prolonging on the call connection in the wireless network, and even with inappropriate parameters, no connection signaling protocol packet can be received by the wireless node.
To clearly illustrate the cited problem, the SIP, wireless local area network (WLAN) and VoIP service are given as an example to describe the signaling protocol design, wireless network and communication service. As shown in FIG. 2, a network node 13 uses a server 12 (SIP proxy) to establish a session with a wireless terminal 15 (SIP user agent or WLAN SIP UA) covered by a base station 11 in the wireless network. The server 12 starts a timer A (Timer A) to count time t1 (i.e., a duration from the time that the server 12 sends the request R1 to the time that the sever 12 receives a response sent by the wireless terminal 15) after sending a connection request R1 (SIP INVITE). If a response is not received in the time t1, the sever 12 re-sends the request R1 and starts the timer A to count a period of time 2×t1. In the SIP specification, it recommends to use exponential retransmission mechanism (i.e., t1, 2×t1, 4×t1, 8×t1, 16×t1, 32×t1, etc.) to reduce the network load, until another timer B (Timer B) reaches a predetermined time.
The SIP specification recommends setting the timer B to a period of time 64×t1 as the predetermined time. However, such a design can cause some problems in the wireless network environment. As shown in FIG. 2, to reduce the power consumption, the wireless terminal 15 may enter a power saving mode (PSM). In the power saving mode, the wireless terminal 15 can listen the beacon sent by the base station 11 every twakeup interval, where the twakeup interval is referred to as a listen interval. When it is found that a packet temporarily stored in the base station 11 is indicated by the beacon of the base station 11, the wireless terminal 15 switches to a normal operating mode in order to download data from the base station 11. In the design of WLAN, the wireless terminal 15 notifies the base station 11 of its sleeping time when determining to enter the power saving mode. Accordingly, the base station 11 starts to temporarily store all packets for the wireless terminal 15 without discarding a packet. However, with limited memory space, the base station 11 can provide limited storage space for each wireless terminal 15 to temporarily store the packets. In this case, if the sleeping time of the wireless terminal 15 exceeds the time provided to temporarily store the packets by the base station 11 and also the network significantly congests, a packet lost may occur. As shown in FIG. 2, it is assumed that the base station 11 can temporarily store the packets for tq interval and notify the wireless terminal 15 every tb interval of whether or not the packets are temporarily stored. Since the twakeup interval is much greater than the tq interval, the SIP INVITE request (R1) may be discarded by the base station 11 after it is sent to the base station 11 for a span. Thus, the wireless terminal 15 cannot receive the request R1 and make the server 12 start a re-transmission mechanism. In this case, a tcallest interval is taken to complete a physical communication establishment, which heavily affects the time required for the communication establishment. In addition, upon the design of inappropriate parameters, the wireless terminal 15 may not receive any call message within 64×t1, thus the communication establishment will fail.
European patent EP1033832 issued to Lucent technologies INC. for a “Wireless data communication system having power saving function” has disclosed a TIM (Traffic Indicator Message) to carry an information to a wireless terminal in order to indicate that a message is temporarily stored in a base station or not. Further, according to the detail of the information, the wireless terminal can determine when to enter a power saving mode. Such a way can effectively overcome the problem of power consumption on the wireless terminal. However, it is suitable for an environment of implementing numerous wireless network access point/base stations due to the custom-designed information fields. Furthermore, such a way cannot effectively overcome the seriously prolonged call setup time caused by the prolonged sleeping time in view of saving the power. Therefore, it is desirable to provide an improved system and method to mitigate and/or obviate the aforementioned problems.